feat: Remove the export from models/index

In order to enhance the cozy-client tree shaking,
we should avoind importing * and exporting.

We had an issue with this PR
#1274

because this PR adds date-fns v2 as dep, but even
if the consuming app doesn't use the concerned model
it was imported.

BREAKING CHANGE: You can not use
import { models } from cozy-client
...
models.files.func()

You have to change it to:
import { func } from cozy-client/dist/models/files